Environ. Sci. Technol. 2005, 39, 6712-6720
Photodegradation of Organic Compounds Adsorbed in Porous Mineral Layers: Determination of Quantum Yields ANDREA CIANI,* KAI-UWE GOSS,* AND R E N EÄ P . S C H W A R Z E N B A C H Swiss Federal Institute for Environmental Science and Technology (EAWAG), Postfach 611, CH 8600 Du ¨ bendorf, Switzerland, and Swiss Federal Institute of Technology (ETH), Zu ¨ rich, Switzerland
Photodegradation is a key process in governing the residence time and fate of many agrochemicals in top soils. However, the basic knowledge of the photolytic transformation reactions of organic chemicals on soil surfaces is still very poor, particularly regarding the quantum yield. In this work we developed a relatively simple model for the quantification of direct photodegradation processes on porous media on the basis of the KubelkaMunk model for radiative transfer. With the help of this model, the quantum yield was determined using two different approaches: (i) the evaluation of the disappearance rate of the compound in the whole layer and (ii) the evaluation of the reflectance change of the doped porous medium during irradiation. The first approach proved to be simplest when applied to optically thin layers where the interference of diffusion kinetics from the nonirradiated part of the layer to the surface is minimal. Here, we report experimental results on the photodegradation of 4-nitroanisole and trifluralin on kaolinite and the first results on goethite. The quantum yield for 4-nitroanisole on kaolinite was found to be on the same order of magnitude as in water, whereas for trifluralin the quantum yield was 10 times smaller than in water. Recommendations for a revision of the presently used OECD/EPA test system are proposed.
Introduction Annually about 5 millions tons of pesticides are applied to crops worldwide (1997, (1)). The drawback of the massive pesticide use is their impact on the environment caused by possible toxic effects. Therefore, in recent decades, interest in the distribution and fate of agrochemicals has permanently increased. Among the degradation processes, photodegradation is important in determining the residence time and fate of many agrochemicals in the environment. For this reason, the investigation of this process is part of the test protocol required by the registration authorities. The relevance of photolysis for the degradation of selected pesticides in aquatic systems and to a certain extent in the gas phase has been documented and has been successfully modeled (2, 3). In contrast, for the photolysis on soils there is no suitable model for the assessment of the kinetics of this process in the environment because of the lack of a proper mechanistic understanding of the underlying phenomena. * Corresponding authors phone: +41 44 823 54 68; e-mail:
[email protected] (K.-U.G.);
[email protected] (A.C.). 6712
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This lack of mechanistic understanding is the result of the difficulty in separating the single factors determining the photodegradation rate constant: the light intensity in the medium, the molar absorption coefficient of the adsorbed compound, and the quantum yield of the photochemical process. The determination of the quantum yield of photochemical reactions in light-absorbing and scattering media such as soils is particularly difficult. To this end, quantification of light intensity in the medium is necessary but difficult because light only penetrates some 10 µm because of the light absorption and scattering on the medium particles (4). Also the molar absorption coefficient of adsorbed compounds, required for the quantum yield determination, is hard to quantify and can only be determined on low lightabsorbing minerals (5). Another problem is that the small light penetration depth causes the formation of a compound concentration gradient within few micrometers of the medium in the course of a photodegradation experiment. This gradient, which cannot be measured in this small space interval, will induce a diffusive transport of the compound toward the irradiated surface. Therefore, only a model that includes diffusion and photolysis kinetics can correctly describe the observed disappearance of the compound (6). In the present work, we advance the modeling of the direct photodegradation processes of compounds adsorbed (i.e., physically bound) in porous media by studying the singular factors affecting the photodegradation kinetics while trying to minimize the interference of diffusion kinetics. The photodegradation rate constant of the compound i at a depth z in the medium, kphoto,i(λ, z), is a function of the light intensity, Itot(λ, z), the molar absorption coefficient, i(λ), and the quantum yield, Φi(λ).
kphoto,i(λ,z) ) f[Itot(λ,z), i(λ), Φi(λ)]
(1)
Note that all three factors are wavelength dependent and that the light intensity is also depth dependent. An appropriate simulation of the disappearance kinetics in a laboratory experiment or even in the field requires an understanding of the functional relationship between kphoto,i(λ,z) and I(λ, z), i(λ), and Φi(λ). In two other publications, we have shown how the light intensity and the molar absorption coefficient can be determined with help of the Kubelka-Munk model (4, 5). In the following, we determine the most difficult parameter, the quantum yield, to improve the current model for photochemical reactions in porous media. The goals of this work were (i) the development of a simple method to determine the quantum yield, (ii) the evaluation of the factors affecting the experimentally observed disappearance kinetics of a compound during irradiation (light intensity, molar absorption coefficient, quantum yield, and diffusion), and (iii) the modeling of photochemical reactions in porous media in order to assess the direct photodegradation of compounds in porous media such as soils.
Theory and General Experimental Approaches Determination of Photodegradation Rate Constants in Porous Media. The photodegradation rate constant of an adsorbed compound (e.g., a herbicide) is a function of the light intensity, molar absorption coefficient, and quantum yield (eq 1). As shown by Ciani et al. (4), light intensity, Itot(λ, z), as a function of depth in porous media composed of mineral particles, can be described with the Kubelka-Munk model (7, 8). The light intensity profile in a layer of thickness d can be calculated with the absorption coefficient k (cm-1) 10.1021/es048096a CCC: $30.25
2005 American Chemical Society Published on Web 07/27/2005
yield, Φi(λ) (mol einstein-1)
∂Ci(t, z) ) ∂t -
∫Φ (λ) 4 ln(10) (λ)C (t, z)I (λ)(ue λ
i
i
i
Rz
0
+ ve-Rz)dλ (8)
If ki , k, the light intensity in the layer will not be significantly influenced by the presence of i (ktot ≈ k) and, hence, Itot(λ, z) will be constant over time. This is a realistic scenario for soils with a typical background concentration of pollutants. When assuming that R, u, and v do not depend on the concentration Ci(t, z) of the adsorbed compound, one can rewrite eq 8 as FIGURE 1. Light fluxes in the depth of a turbid medium (of thickness d) following the Kubelka-Munk model. The sum of the downward flux, I(λ, z), and the upward flux, J(λ, z), gives the total light intensity in the layer at a depth z, Itot(λ, z). The remitted and transmitted light intensity is given by multiplying the reflectance (R) or transmittance (T) of the layer with incoming light intensity I0(λ). and the scattering coefficient s (cm-1) of the medium (4). The total light intensity in the medium at a depth z, Itot(λ, z), described as the sum of the two light fluxes, I(λ, z) and J(λ, z) (downward and upward, Figure 1), can be expressed by the following equation (4):
Itot(λ, z) ) 2I0(λ)(ueRz + ve-Rz)
(2)
where u and v are defined as (9)
u) v)
(β - 1)e-Rd 2 Rd
(1 + β) e
- (1 - β)2e-Rd
(1 + β)eRd 2 Rd
(1 + β) e
- (1 - β)2e-Rd
(3)
(4)
and I0(λ) (einstein cm-2 s-1 nm-1) is the light intensity at the surface, R ≡ xktot(ktot+2s), β ≡ xktot/(ktot+2s), and ktot ) k + ki (cm-1) is the total absorption coefficient accounting for light absorption by the mineral (k) and by the adsorbed compound i (ki, see below). Note that ki, k, s, R, β, u, and v are wavelength dependent. For the determination of the photodegradation rate constant it is also necessary to quantify the number of photons absorbed by the compound per unit time and volume, Ni(t, λ, z), in a slice of infinitesimal thickness positioned at a depth z in the layer. This quantity is proportional to the total light intensity, Itot(λ, z), in the medium and to the light absorption coefficient of the adsorbed compound, ki (cm-1) (10)
Ni(t, λ, z) ) kiItot(λ, z)
(5)
The coefficient ki is proportional to the concentration Ci(t, z) (mol cm-3) and to the molar absorption coefficient i(λ) (cm2 mol-1) (5)
ki(t, λ, z) ) 2 ln(10)i(λ)Ci(t, z)
(6)
By inserting eqs 2 and 6 into eq 5 and integrating over the wavelength range of interest, one gets an equation describing the total quantity of photons absorbed by the compound i per unit time and volume at a depth z
Ni(t, z) )
∫ 4 ln(10) (λ)C (t, z)I (λ)(ue λ
i
i
0
Rz
+ ve-Rz)dλ (7)
Finally, we receive an expression for the photodegradation rate of compound i at a given depth z in the layer, ∂Ci(t, z)/∂t, which corresponds to Ni(t, z) multiplied by the quantum
∂Ci(t, z) ) -kphoto,i(z)Ci(t, z) ∂t
(9)
with the depth dependent pseudo first-order photodegradation rate constant
∫Φ (λ) (λ)I (λ)(ue
kphoto,i(z) ) 4 ln(10)
λ
i
i
0
Rz
+ ve-Rz) dλ (10)
For practical applications, as in the following, we assume an average quantum yield across all wavelengths, Φi. Quantum Yield Determination. The average quantum yield of the adsorbed compound, Φi, is the only unknown parameter of eq 10 that has to be determined to fully describe a direct photodegradation process in a porous layer. For all other parameters, we have already provided methods for their determination in (4, 5). Two experimental approaches were used in this work for the determination of the quantum yield. The first approach is based on the irradiation of mineral layers of defined thickness d that are homogeneously doped with the compound. After irradiation, the total amount of the remaining compound in the whole layer is measured at different time intervals. This kind of experiment is also the method proposed by the EPA and OECD (11, 12); however, it has an insufficient evaluation procedure. For these experiments two evaluation methods, A and B, are proposed here. The second approach is spectroscopic and is based on a nonintrusive diffuse reflectance measurement. The time dependent decrease of the parent compound concentration is determined by observing the change in the reflectance of a doped layer during the irradiation. Two evaluation methods, C and D, were developed based on this spectroscopic approach. Method A: Coupled Photodegradation and Diffusion. Depending on the layer thickness of the porous medium, the diffusion kinetics of the compound will also affect the observed disappearance rate of i (6). Thus, to fully describe the observed disappearance rate in an irradiated porous layer, eq 9 has to be combined with a term describing the diffusive transport of the compound i
∂Ci(t, z) ∂2Ci(t, z) - kphoto,i(z)Ci(t, z) ) Deff ∂t ∂z2
(11)
where kphoto,i(z) is defined in eq 10 and Deff (cm2 s-1) is the effective diffusion coefficient of the compound i in air (assuming an air-dry porous layer). Deff depends on the diffusivity of the compound in air, its sorption to the solid matrix, and a tortuosity factor (6, 13). At the upper and lower boundaries, we take Deff ) 0 for z < 0 and z > d because diffusion is restricted by the glass plates (6). Unfortunately, the photodegradation rate constant kphoto,i(z) will change drastically within a small depth interval because of the limited VOL. 39, NO. 17, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Contour plot of the critical layer thickness, d (in µm), dependent on the absorption coefficient, k, and scattering coefficient, s, required for reaching a transmittance T of 0.05.
FIGURE 2. Photodegradation of a hypothetical compound (Ei ) 16 × 106 cm2 mol-1, Φi ) 0.001) and light regime (k ) 200 cm-1, s ) 5000 cm-1, I0 ) 5 × 10-9 einstein cm-2 s-1) expressed as ratio of the amount of compound Mi to the initial amount Mi (t ) 0) for (a) an optically thick layer with d ) 70 µm at different effective diffusion coefficients (number near the lines correspond to -log(Deff/cm2 s-1)) and (b) an optically thin layer with with d) 15 µm. The upper curve is for the case of no diffusion in the layer and the lower curve corresponds to a very rapid diffusion. light penetration in the porous medium. Experimentally, only the disappearance kinetics of the compound i in the whole layer can be determined (6). By integrating eq 11 over the layer thickness (from z ) 0 to d) and multiplying with the surface area of the layer, A, one gets an equation for the disappearance of the compound i in the whole layer, Mi(t) (mol), for low concentrations of i in order to avoid an inner filter effect (6)
∂Mi(t) )A ∂t
∫
d
0
[
2
]
∂ Ci(t, z) Deff - kphoto,i (z)Ci(t, z) dz ∂z2
9
Σ kphoto,i )
4Φi ln(10) d
∫ ∫ (λ)I (λ)(ue
(12)
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 17, 2005
d
0
λ i
0
Rz
+ ve-Rz)dλdz (13)
By inverting the integrals and solving them over the depth z ) 0 to d one obtains Σ kphoto,i ) 4Φi ln(10) d
∫
This equation can be used to fit experimental data and determine Φi (included in kphoto,i(z)) and Deff. To this purpose the impinging light intensity, I0(λ), the molar absorption coefficient of the compound, i(λ), the absorption coefficient k, the scattering coefficient s of the medium, and the initial compound concentration, Ci(t ) 0, z), have to be known. To improve the accuracy of the fitting parameters Φi and Deff, it is necessary to perform a simultaneous fit of a series of experimental curves at various layer thicknesses (6). Note that the effective diffusion coefficient Deff could also be estimated a priori (6, 14, 15). Method B: Quantum Yield Determination for Optically Thin Layers. The overall observed photodegradation rate constant for an optically thick layer can be strongly influenced by the diffusion kinetics of the compound in the layer as described above (method A and Figure 2a). The disappearance curve for a layer where “no diffusion” occurs is completely different from that for a “rapid diffusion” 6714
(infinitely fast diffusion) layer. Hence, a simple method for the quantum yield determination, independent of the diffusion process, is an experiment with optically thin media because, in this case, light penetrates through a great part of the layer and only a small concentration gradient can build up. Because of the small concentration gradient the observed disappearance curves for no diffusion and rapid diffusion will fall much closer together. Figure 2b shows that a layer with a transmittance higher than 0.05 provides such conditions because, in this case, the two extreme disappearance curves are similar up to 70% photodegradation independent of the effective diffusion coefficient. Therefore, for optically thin layers having transmittance values of at least 0.05 (at those wavelengths that are effective for the phototransformation process), one can approximate the actual photodegradation curve with the rapid diffusion layer curve. In this case eq 10 can be simplified by integrating kphoto,i(z) from z ) 0 to d and dividing the result by the layer thickness d. In this manner one obtains Σ the photodegradation rate constant kphoto,i which is observed for the whole rapid diffusion layer
λ
i(λ)I0(λ) (u(eRd - 1) + v(e-Rd - 1))dλ (14) R
Solving eq 14 for Φi provides an equation for the determination of the quantum yield from the observed disappearance rate constant in an optically thin porous layer without Deff
Φi )
Σ d kphoto,i
∫
4 ln(10)
i(λ)I0(λ) (u(eRd - 1) + v(e-Rd - 1)) dλ R (15)
λ
It is necessary to prepare very thin layers in order to obtain the required layer transmittance of >0.05. Figure 3 shows the layer thickness required for a transmittance of 0.05 as a function of the absorption coefficient k and scattering coefficient s of the porous medium. Note that if diffusion is relatively fast compared to the photodegradation process, even a rather thick layer can be approximated as a rapid diffusion layer (see curve in Figure 2a for Deff ) 10-8 cm2 s-1). Furthermore, note that the shape of the curves rapid diffusion and no diffusion is the same for a given product of Φi and irradiation time.
Method C: Spectroscopic Determination of the Quantum Yield. The second experimental approach is the spectroscopic determination of the quantum yield by observing the change in the diffuse reflectance of a doped layer during a photochemical reaction. This change is a function of the decreasing concentration of the investigated compound and the increasing concentration of reaction products that have, in general, different molar absorption spectra. From the initial slope of the measured diffuse reflectance versus irradiation time, the reaction quantum yield can be derived with a method similar to that described by Gade and Porada (10, 16). A detailed description of this method can be found in the Supporting Information. This relatively simple method has the following limitations: (i) limited precision because only a part of the reflectance curve is used for the calculation, (ii) if there is more than one product formed during irradiation this method cannot be used, and (iii) any diffusion of i and of the product p decreases the precision of the calculation. Note, though, that the irradiation time can be kept very short using a high-intensity laser as light source and, therefore, diffusion will cause fewer problems than for method A. Method D: Extended Spectroscopic Approach. To overcome the shortcomings of method C, another evaluation method for the spectroscopic measurements can be used. The infinite reflectance throughout the whole irradiation time is simulated by using the quantum yield as a fitting parameter (see Supporting Information for more details). This evaluation method has the advantages that (i) the information of the whole experiment is used for the determination of the quantum yield, (ii) diffusion can be accounted for by adding diffusion equations for compounds i and p, (iii) a good agreement between fitted and experimental curve will indicate the absence of problems that would occur with more than one product, and (iv) this method allows the use of the infinite reflectance as well as both reflectance and transmittance data for the determination of the quantum yield (method C is only applicable with the infinite reflectance).
Materials and Description of Experimental Systems The photodegradable probe compounds used are 4-nitroanisole (PNA) and trifluralin (Riedel-de Hae¨n). Kaolinite (China Clay Supreme from English Clays Lovering Pochin & Co. Ltd, St.Austell/Cornwall, BET surface: 12 m2 g-1) and goethite (Bayferrox 910 standard 86, Bayer, BET surface: 16 m2 g-1) were used as porous media. Photodegradation Experiments of PNA and Trifluralin on Kaolinite. For methods A and B, the photodegradation kinetics of PNA and trifluralin on kaolinite layers from a previous work of Balmer et al. (6) were used. The absorption coefficient k and the scattering coefficient s of kaolinite were taken from ref 4, and the molar absorption coefficient of PNA and trifluralin on kaolinite were taken from ref 5. The spectrum of the xenon lamp used for irradiation is given in Figure S1 of the Supporting Information. For methods C and D, laser flash photolysis experiments were conducted with kaolinite layers produced according to ref 6. A Nd:YAG laser (Brilliant B, Quantel, Les Ulis Cedex, France) with frequency multiplying crystals (3x: 355 nm and 4x: 266 nm) was used as a light source. Alternatively, the 355 nm laser light was used to pump an optical parametric oscillator (OPO, Opotek, Carlsbad CA) to produce wavelengths between 410 and 690 nm. The energy of the laser light (7.8-60 mJ per pulse) was measured using a Gentec Duo Energy meter and a ED500 joulemeter head (both Gentec, Sainte-Foy, Canada). The pulse rate of the laser was set between 1 and 10 Hz depending on the experiment. The laser beam is enlarged with a lens and passes through a frame to irradiate a defined rectangular area of 1.4 cm × 2.2 cm on the layer. The spectral diffuse reflectance of the irradiated layer was recorded as a function of the irradiation time with
an UVIKON 860 spectrophotometer equipped with a 9 cm integrating sphere (wavelength range, 275-500 nm; scanrate, 100 nm min-1; bandwidth, 2 nm). The quantum yield was obtained by fitting the data with eqs S5 and S6 for method C and eqs S1-S4 for method D (see Supporting Information). For comparison, photodegradation experiments were performed by irradiating the samples with UV lamps (Philips Actinic Blue TL 20W/05, see Figure S1, Supporting Information). Photodegradation Experiments of PNA on Goethite. The experimental procedure was similar to that of Balmer et al. (6). Eight Pyrex glass plates with air-dry goethite layers (thickness d ) 15 µm) were spiked homogeneously with 500 µL of a PNA solution in hexane to reach a concentration of 1.7 µmol g-1. The layers were then covered with a Pyrex glass and exposed to a xenon lamp (Figure S1, Supporting Information) for different time intervals. A dark control experiment was carried out with a layer covered with a red film (cut off wavelength: 560 nm). After the sample was irradiated, PNA was extracted from goethite with methanol and analyzed according to ref 6. For the determination of the quantum yield, method A (eq 12) was used. The required absorption coefficient k and the scattering coefficient s of goethite were previously determined (4) and the molar absorption coefficient of PNA on goethite was assumed to be similar as on kaolinite (see above).
Results and Discussion Photodegradation of PNA on Kaolinite. A fit of the experimental data for the photodegradation of PNA on kaolinite layers of various thickness from Balmer et al. (6) was performed with our photodegradation model described by eq 12 (method A). The fitting curves, determined with the “fminsearch” function of Matlab, describe the measured values well (Figure 4) with a resulting quantum yield of Φi ) 1.7 × 10-4 and a diffusion coefficient Deff ) 5.6 × 10-10 cm2 s-1. Similar quantum yield values (from 1.4 × 10-4 to 1.8 × 10-4, see Table 1A) were obtained using the much simpler method B (eq 15), demonstrating the feasibility of deriving the quantum yield simply from the initially observed phoΣ todegradation rate (≈ kphoto,i ) on optically thin layers (the three thinnest layers in Figure 4 were used for the calculation). This is illustrated in Figure 4 where curves for photodegradation without diffusion and with very rapid diffusion are shown. These two curves represent the limiting cases for the actual photodegradation curve. For thin layers, the actual fit curve (see d ) 11.3 µm in Figure 4) is identical to the rapid diffusion curve and is similar to the no diffusion curve. By increasing the layer thickness the difference between these two limiting curves becomes more evident, and the actual fit is positioned between them because light can only penetrate to some extent in the layer and the diffusion kinetics of the compound becomes important. This means that the photodegradation kinetics of an optically thin layer can be treated as a rapid diffusion layer because diffusion kinetics has a negligible impact on the observed photodegradation curve (as shown in Figure 4a-c). For the spectroscopic determination of the quantum yield (Methods C and D), rather thick layers (d ) 50 µm, to obtain the maximal reflectance signal) of kaolinite, doped with 23 µmol g-1 PNA, were irradiated with a pulsed laser at 355 nm, near to the absorption maximum of PNA on kaolinite (320 nm). To test the effect of different pulse frequencies of the laser, experiments were performed at pulse frequencies of 1, 2, and 10 Hz and different pulse irradiation energies (59.5, 38.6, and 7.8 mJ, respectively) to have similar irradiation times. No significant difference in the results was observed confirming that the frequency of the laser pulses does not affect the kinetics. After an irradiation of about 2 × 10-4 einstein cm-2, a steady-state value for the reflectance was VOL. 39, NO. 17, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Quantum Yield for the Degradation of PNA and Trifluralin on Air-dry Kaolinite Determined with Different Methods A. PNA method of evaluation method A method B
method C method D
Φi
experimental condition
1.7 × 10-4 1.4 × 10-4 1.6 × 10-4 1.8 × 10-4 6.8 × 10-4 9.0 × 10-4
xenon lamp, Deff ) 5.6 × 10-10 cm2 s-1 xenon lamp, d ) 11.3 µm xenon lamp, d ) 22.5 µm xenon lamp, d ) 46 µm laser 355 nm, R∞(λ ) 400 nm) laser 355 nm, R∞(λ ) 400 nm), Deff ) 5.6‚10-10 cm2 s-1 Hg Lamp 366 nm (17)
literature 2.9 × 10-4 (in water)
B. Trifluralin method of evaluation
2.2 × 10-4
method B
2.3 × 10-4 1.9 × 10-4 1.6 × 10-4 1.0 × 10-4 0.7 × 10-4 0.9 × 10-4 1.6 × 10-4
method C
method D literature (in water)
FIGURE 4. Fitting of the photodegradation of PNA on kaolinite measured by Balmer et al. (6) (b) with eq 12. The solid line corresponds to the fitted model with Φi )1.7 × 10-4 and Deff ) 5.6 × 10-10 cm2 s-1. The dashed lines are calculated for the same quantum yield but for no diffusion and very rapid diffusion, respectively. These two curves cover the space in which the photodegradation curve can be located. The reflectance and transmittance values (R and T) for pure kaolinite at 320 nm are displayed. reached, indicating that the photochemical reaction within the information depth (maximal depth from which the reflectance signal originates) of the light at 355 nm in kaolinite (about 15 µm (4)) is complete (Figure 5). The change of the complete spectrum during irradiation is depicted in Figure 6. Assuming that there is only one product from the photodegradation of PNA, the molar absorption coefficient, p(λ), of the product can be calculated with the reflectance at the end of the experiment. With the initial slope method proposed by Gade (Method C) a quantum yield of 6.8 × 10-4 is obtained using an observation wavelength of 400 nm (a calculation with the reflectance values at 355 nm is not possible because the 355 nm signal is constant over time, see Figure 6). The two isosbestic points observed at 305 and 355 nm (dashed lines in Figure 6) confirm that there is either only one product formed or several products with similar spectra. By simulating the reflectance versus irradiation time curve (method D, quantum yield of 9.0 × 10-4) the validity 6716
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Φi
method A
experimental condition
2.0 × 10-3
xenon lamp, Deff ) 8.9 × 10-10 cm2 s-1 xenon lamp, d ) 22.5 µm xenon lamp, d ) 46 µm laser 420 nm laser 440 nm laser 450 nm laser 460 nm laser 440 nm, 440 and 500 nm, Deff ) 8.9‚10-10 cm2 s-1 Hg lamp 366 nm (18)
1.4 × 10-3
polychromatic 310-410 nm (19)
FIGURE 5. Laser flash photolysis of PNA on kaolinite at 355 nm (2 Hz). The fitting curve, calculated with method D, describes the reflectance values at 400 nm very well. of this assumption could be further confirmed (Figure 5) by a good agreement between simulated and measured data. This is on the same order of magnitude as the value calculated with method C. For method D, the diffusion of the compounds in the layer with an effective diffusion coefficient of 5.6 × 10-10 cm2 s-1 has been included (value taken from method A). The laser irradiation experiments (methods C and D) lasted about 6 h and can even be shorter if the reflectance is measured online, whereas for methods A and B, 35 to 250 h were needed. All values for the quantum yield as well as the corresponding experimental and evaluation conditions are summarized in Table 1. The values determined with methods A and B are significantly smaller (4-5 times) than the values determined with the laser experiment. This might be the result of the different reaction pathways that may occur at different irradiation wavelengths. It is possible that monochromatic irradiation with a laser will induce one reaction pathway, while the polychromatic xenon lamp used by Balmer et al.
FIGURE 6. Laser flash photolysis of PNA on kaolinite at an irradiation wavelength of 355 nm and 10 Hz frequency. The arrows indicate the direction of decrease/increase of the reflectance during the irradiation. The dashed curve corresponds to the reflectance of pure kaolinite.
FIGURE 7. (a) Spectral change of the transmittance of a thin layer of kaolinite (d ) 4.2 µm) doped with PNA and irradiated with a UV lamp. The dashed curve corresponds to the transmittance spectrum of pure kaolinite. (b) The transmittance value at 320 and 400 nm reach a limit after 400 min of exposure. The dashed lines are only a visual help and not a model. will promote several pathways with different quantum yields. As a result the average quantum yield can be smaller than that at a fixed wavelength. Evidence for this explanation comes from the observation that the spectral change of an UV-irradiated thin kaolinite layer doped with PNA (Figure 7) is different from that obtained with laser irradiation (Figure 6). With the UV lamp irradiation, there is no clear isosbestic point but instead the crossing point of the transmittance spectra is moving toward a longer wavelength during irradiation. This suggests that various photoreaction products with different spectra were formed in the experiment with the polychromatic irradiating UV lamp. Hence, the photoreaction pathway did depend to some extend on the wavelength of irradiation in the case of PNA on kaolinite. Note that the average quantum yield determined with the
FIGURE 8. Fitting of the photodegradation of trifluralin on kaolinite measured by Balmer et al. (6) (b) with eq 12. The solid line corresponds to the fitted model with Φi ) 2.2 × 10-4 and Deff ) 8.9 × 10-10 cm2 s-1. The dashed lines are calculated for the same quantum yield but for no diffusion and very rapid diffusion, respectively. These two curves circumscribe the space where the photodegradation curve can be located. The reflectance and transmittance values (R and T) for pure kaolinite at 430 nm are displayed. data from Balmer et al. (method A and B) is 40% smaller than the literature value in water (2.9 × 10-4) reported by Dulin and Mill (17) for an irradiation wavelength of 313 nm. Photodegradation of Trifluralin on Kaolinite. Kinetic data from Balmer et al. (6) were fitted with method A yielding the degradation curves shown in Figure 8. The fit results in a quantum yield of Φi ) 2.2 × 10-4 and an effective diffusion coefficient of 8.9 × 10-10 cm2 s-1. The quantum yield was also calculated using method B from the data of the two thinnest kaolinite layers (22.5 and 46 µm) for which the assumption of a layer with rapid diffusion is justified. The results are identical to those calculated with method A (see Table 1B), confirming again, as for PNA, the fact that method B gives reliable results on the basis of a much smaller set of experimental data than method A. For the spectroscopic determination of the quantum yield rather thick layers (d ) 50 µm) of kaolinite doped with 4.4 µmol g-1 trifluralin were irradiated with a pulsed laser using VOL. 39, NO. 17, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 9. (a) Spectral change of the infinite reflectance of a layer of kaolinite doped with trifluralin and irradiated with a laser irradiation at 440 nm. (b) Similar experiment conducted with a UV lamp. The dashed line corresponds to the spectral reflectance of pure kaolinite.
FIGURE 10. Simultaneous fit (method D) at 440 and 500 nm for the reflectance of a kaolinite layer doped with trifluralin and irradiated with a laser at 440 nm (pulse frequency: 10 Hz). four single wavelengths (separate experiments at 420, 440, 450, and 460 nm) near to the absorption maximum of trifluralin on kaolinite (430 nm). For trifluralin, it thus became possible to test the wavelength dependence of the quantum yield. The change in the reflectance spectrum for the four irradiation wavelengths is very similar (an example is given in Figure 9a). The calculated quantum yield values (method C) for these different wavelengths range between 0.7 × 10-4 and 1.6 × 10-4 (Table 1B) with no clear wavelength dependence (a 2-fold variation is not significant for such experiments). With method D, the spectral reflectance change during the irradiation time was simulated, yielding a similar quantum yield as that with method C. For the calculation, Deff ) 8.9 × 10-10 cm2 s-1, obtained with method A, was used. The fitting curve describes the measured data well (Figure 10). As in the experiments with PNA, a different spectral change for the irradiation of trifluralin with an UV lamp and a laser (440 nm) were found, indicating different reaction pathways (see Figure 9b). In the laser experiment, an 6718
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FIGURE 11. Photodegradation of PNA on goethite layer of thickness d ) 15 µm with a xenon lamp, fitted with eq 12: (a) fitting with Φi ) 3.3 × 10-3 and Deff ) 1.3 × 10-12 cm2 s-1; (b) fitting with Φi ) 9.6 × 10-4 and Deff ) 6 × 10-11 cm2 s-1. The reflectance and transmittance at 320 nm are R ) 0.04 and T ) 0. The light penetration depth at a wavelength of 320 nm (maximum of Ei of PNA) is 0.2 µm (4). isosbestic point at 353 nm is observed, whereas the spectral reflectance during the UV-lamp experiment does not show a clear isosbestic point. The quantum yield values determined with methods A and B are very similar to the values determined for fixed wavelengths with methods C and D with no dependence on the light source used. In contrast to PNA, all values for trifluralin adsorbed on kaolinite are about 1 order of magnitude smaller than the quantum yields reported in the literature for its photodegradation in water (Table 1B). This is probably the result of the interaction of one NO2 group of trifluralin with the clay surface as Margulies et al. (20) have proposed for trifluralin on montmorillonite. They suggested that this adsorption causes a steric hindrance of the cyclization step of the photochemical reaction. Photodegradation of PNA on Goethite. The photodegradation of PNA on goethite is much slower than on kaolinite for similar layer thicknesses (compare Figure 11 with Figure 4a and b). Because of the high light absorbance (and consequent low reflectance) of goethite, particularly below 500 nm, it was not possible to see an absorbance signal of PNA adsorbed on goethite. Therefore, any spectroscopic determination (methods C and D) of PNA on goethite was not possible. Because of the difficulty obtaining the optically thin layers needed for method B, only method A was applicable for the determination of the quantum yield on goethite. The molar absorption coefficient i(λ) of PNA on goethite was not measurable but was assumed to be similar to that on kaolinite. This assumption is reasonable because the molecular interactions that cause adsorption of PNA on the two minerals should be similar (21, 22). Furthermore, the maximal positive solvatochromic shift of PNA is 27 nm (from cyclohexane (23) to kaolinite). For these reasons the spectrum shift between PNA on kaolinite and PNA on goethite
is probably very low. The decrease of the PNA concentration was modeled with eq 12. Because this experiment was performed only for one layer thickness and the light penetrates only a small part into the layer (1.3% of the total layer thickness), it was not possible to achieve a unique fit of the data. In Figure 11, two alternative fits are depicted which can both explain the experimental values. The better fit, shown in Figure 11a, gives a quantum yield of Φi ) 3.3 × 10-3 and a very small effective diffusion coefficient, Deff ) 1.3 × 10-12 cm2 s-1, which seems unrealistic compared to the value found for kaolinite. The photodegradation under this condition is clearly a diffusion-limited process and the measured photodegradation curve lies between the no diffusion and the rapid diffusion curves. The second possible fit found has a quantum yield of Φi ) 9.6 × 10-4 and an effective diffusion coefficient of Deff > 6 × 10-11 cm2 s-1. For this second fit, the Deff is a minimal value because by increasing the Deff value no change of the fitting curve is observed (as can be seen in Figure 11b, the fitting curve overlaps with the rapid diffusion curve). In fact, in this case, the photodegradation kinetics of PNA is not controlled by the diffusion kinetics but rather limited by the relatively low photodegradation rate constant. As shown in this experiment, it is very difficult to determine the quantum yield of a reaction on highly absorbing media with one single experiment because optically thin layers cannot be produced. Despite the nonunique fitting of the measured data, the fitted quantum yield can be estimated to lie in the range of 9.6 × 10-4-3.3 × 10-3, which is higher than the values determined in water and on kaolinite (see Table 1A). Therefore, the observed slow photodegradation of PNA on goethite must mostly be caused by the small penetration depth of light rather than differences in the quantum yield. Comparison of the Methods Used. Generally, the methods used here allowed us to determine at least the order of magnitude of the quantum yield. For more precise measurements, the uncertainty in the input data (k, s, i(λ), and I0(λ)) would have to be reduced significantly. Note, however, that the quantum yield can be determined more accurately if the experimental system is similar to that used to determine the parameters k, s, and i(λ). The spectroscopic approach (methods C and D) for the quantum yield determination is interesting because it provides information about the possible reaction pathways (different diffuse reflectance spectra) in addition to the quantum yield. This approach is experimentally convenient because the whole irradiation experiment can be performed with a single porous layer and within short time. However, the numerical evaluation of the reflectance data, especially for method D, is extremely laborious and not adequate for a simple quantum yield determination. On the other hand the quantum yield determination with methods A and B is based on the analytical measurement of the compounds’ disappearance in the whole layer. Method B is the simplest method for the determination of quantum yield on porous media because only the disappearance kinetics for a single layer thickness (optically thin layer) must be determined and a rather simple spreadsheet software can be used for the evaluation (eq 15). For photodegradation processes on soils, or in general on thicker porous media, it is necessary to couple the photodegradation process with the diffusion kinetics of the compound in the layer. Evaluation method A also considers the diffusion kinetics (calculations with eq 12 are more complicated than method B) and can therefore be applied to a wide set of experimental and ambient conditions. Method A can be used for the simultaneous determination of quantum yield and effective diffusion coefficient if kinetic data for various layer thicknesses are available. This simulation can
also be used to predict the photodegradation of compounds in soils under ambient conditions for various scenarios such as different initial concentration profile, quantum yield, or light absorption and scattering coefficient. This work demonstrates the possibilities and limits of various experimental procedures used to assess photodegradation in porous media. The OECD/EPA test guidelines for the photodegradation of pesticides on soil should be revised with these findings in mind to produce experimental results that are better comparable and allow a more meaningful evaluation. (1) The experiments should be conducted in a manner such that only photolytic reactions determine the observed reaction rate (e.g., method B presented here). The kinetics of phototransformation experiments according to the current OECD/EPA guidelines with a 2 mm thick soil layer will be strongly influenced or even dominated by diffusion. Such results are meaningless and cannot be interpreted for regulatory purposes especially because major factors (e.g., moisture, thickness of the layer, and specific surface area of the minerals) affecting the diffusion velocity are not at all controlled. (2) Experiments should be conducted with one standard soil (or at least with soils that do not differ substantially in their light penetration behavior) to have a comparable experimental setup for various chemicals. Otherwise different degradation kinetics could erroneously be attributed to different chemical behaviors although they were only the result of the different light intensities in the soils.
Acknowledgments We thank Thomas Nauser for his help with the laser flash photolysis experiments, Prof. Jo¨rg Waldvogel for his support with numerical calculations, and Silvio Canonica, Andreas Gerecke, and two anonymous reviewers for their critical corrections.
Supporting Information Available Details are provided about (1) spectra of the lamps used, (2) absorption and scattering coefficient of kaolinite, (3) molar absorption coefficient of PNA and trifluralin adsorbed on kaolinite, and (4-5) calculations with methods C and D. This material is available free of charge via the Internet at http:// pubs.acs.org.
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Received for review December 2, 2004. Revised manuscript received May 20, 2005. Accepted June 23, 2005. ES048096A
